Simple interest is calculated only on the original principal. It creates a straight-line relationship between principal, rate and time, which makes it useful for teaching, checking certain agreements and estimating products explicitly described as simple interest.
The simple-interest formula
Open the Simple Interest Calculator and enter principal, annual rate and time. The standard formula is:
Interest = principal × annual rate × time in years
The total value is principal plus interest. Convert the percentage to a decimal in a manual calculation: 6% becomes 0.06.
For example, 5,000 at 6% simple interest for two years produces 600 interest: 5,000 × 0.06 × 2. The total is 5,600, assuming no fees, repayments or other rules.
Match the time unit to the rate
An annual rate requires time expressed in years. Six months is 0.5 years. Ninety days may be calculated using a 365-day or 360-day convention depending on the agreement. A calculator cannot choose the contractual day-count method unless the product specifies it.
Do not multiply a monthly rate by years without converting the units. Consistent units are essential.
Simple interest is not the same as an instalment loan
Many consumer loans are amortised: payments reduce the balance, and interest is calculated on the outstanding amount. The simple-interest formula applied once to the original principal may not reproduce that schedule. Use the Loan Calculator for an equal-payment amortising estimate.
Compound interest also differs because earlier interest can generate later interest. Compare the Compound Interest Calculator when growth compounds periodically.
Fees can outweigh the clean formula
Origination charges, account fees, insurance, taxes, late charges and early-payment rules are outside the three-variable formula. When comparing borrowing, use the annual percentage rate and total repayment disclosures where available rather than assuming simple interest represents the full cost.
For savings, verify whether the advertised number is a nominal rate or an annual percentage yield. APY already reflects a compounding convention and should not be treated as a simple nominal rate without adjustment.
Questions people ask
Can simple interest be charged daily?
Yes. An agreement can calculate daily simple interest on an applicable principal balance. The contract's day-count and payment rules control the result.
Does making a payment reduce simple interest?
It may if interest is calculated on a declining principal, but that is different from applying the basic formula once to the original amount. Check the agreement.
Why is my lender's result higher?
Fees, different date conventions, payment timing or an amortisation method may explain the difference.
Is simple interest always cheaper than compound interest?
Not automatically. The principal, rate, term, fees and payment structure must all be compared.
Useful next tools
- Compound Interest — Grow savings with regular contributions over time.
- Loan Calculator — Payment, interest and payoff for any personal or business loan.
- Savings Goal Planner — How much to set aside each month to hit a target.
- Percentage Calculator — Percent of, is-what-percent, and percentage change.
- Mortgage Calculator — Monthly payment, total interest and full amortisation schedule.
The result is an educational estimate. The financial contract determines actual interest, dates, charges and repayment obligations.
Try the tools from this guide
Keep reading
How Compound Interest Grows Savings Over Time
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How to Calculate Loan Payments, Interest and Borrowing Cost
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